Biallelic Mutations in MYPN, Encoding Myopalladin, Are Associated with Childhood-Onset, Slowly Progressive Nemaline Myopathy

Abstract

Nemaline myopathy (NM) is a common form of congenital nondystrophic skeletal muscle disease characterized by muscular weakness of proximal dominance, hypotonia, and respiratory insufficiency but typically not cardiac dysfunction. Wide variation in severity has been reported. Intranuclear rod myopathy is a subtype of NM in which rod-like bodies are seen in the nucleus, and it often manifests as a severe phenotype. Although ten mutant genes are currently known to be associated with NM, only ACTA1 is associated with intranuclear rod myopathy. In addition, the genetic cause remains unclear in approximately 25%-30% of individuals with NM. We performed whole-exome sequencing on individuals with histologically confirmed but genetically unsolved NM. Our study included individuals with milder, later-onset NM and identified biallelic loss-of-function mutations in myopalladin (MYPN) in four families. Encoded MYPN is a sarcomeric protein exclusively localized in striated muscle in humans. Individuals with identified MYPN mutations in all four of these families have relatively mild, childhood- to adult-onset NM with slowly progressive muscle weakness. Walking difficulties were recognized around their forties. Decreased respiratory function, cardiac involvement, and intranuclear rods in biopsied muscle were observed in two individuals. MYPN was localized at the Z-line in control skeletal muscles but was absent from affected individuals. Homozygous knockin mice with a nonsense mutation in Mypn showed Z-streaming and nemaline-like bodies adjacent to a disorganized Z-line on electron microscopy, recapitulating the disease. Our results suggest that MYPN screening should be considered in individuals with mild NM, especially when cardiac problems or intranuclear rods are present.

Keywords: MYPN; congenital myopathy; intranuclear rod myopathy; nemaline myopathy; whole-exome sequencing.

Figure 1

Pedigrees of Families Harboring MYPN Mutations (A) Families with biallelic MYPN mutations. Black squares and black circles indicate affected male and female individuals, respectively. Arrows indicate probands, and asterisks indicate individuals analyzed by WES. Minus signs indicate that the mutation was not found. (B) MYPN mutations identified in this study. The upper panel shows a schematic presentation of MYPN structure. Gray and black boxes indicate the coiled-coil domain and Ig domain, respectively. The N-terminal CARP-binding region, nebulin-binding region, and C-terminal α-actinin-binding region are presented. The lower panel shows the positions of the identified mutations along with altered proteins.

Figure 2

Pathologies of Biopsied Muscle from the Studied Individuals For muscle histology, the muscle samples were frozen in liquid-nitrogen-cooled isopentane and stored at −80°C. They were then sectioned at a thickness of 10 μm and exposed to a battery of routine histochemical stains, including hematoxylin and eosin (H&E), modified Gomori trichrome (mGT), and NADH-TR. Regarding the method of electron microscopy (EM), see the Figure S7 legend. Because we could not obtain standard EM samples of the muscles from individual 4, frozen sections were fixed with 2.5% glutaraldehyde in cacodylate buffer and subjected to standard epon-embedded EM block preparation. (A) Sections stained with H&E show marked variation in fiber size, including replacement of interstitial connective and fat (F) tissue in individual 1, who is wheelchair bound. The other three individuals showed mild to moderate variation in fiber size with scattered small angular fibers representative of mild muscle changes. No apparent degenerative changes were recognized. Scale bar represents 50 μm. (B) mGT staining of biopsied muscle. Scattered fibers containing dispersed to aggregated darkly stained nemaline bodies in the cytoplasm (arrows) were found in all subjects. Most of the bodies were round and not rod-like in shape. Intranuclear rods (encircled) with a fine string-like appearance (stained red) were seen in individuals 1 and 4. Scale bar represents 10 μm. (C) EM confirmed the presence of nemaline bodies (arrow) with the same electron density as the Z-line in individuals 2 and 3. Most of the bodies were short and rarely longer than the length of one sarcomere. Note the relatively well-preserved myofibrils. An intranuclear rod was seen in individual 4; it had a slightly lower electron density than intracytoplasmic nemaline bodies but a lattice-like structure (inset). Scale bar represents 1 μm.

Figure 3

Immunohistological and Protein Analyses of the Studied Individuals (A) Immunohistochemical staining was performed on 10-μm-thick frozen sections of muscles with a rabbit polyclonal anti-MYPN antibody, mouse monoclonal anti-α-actinin (EA-53; Sigma-Aldrich), and DAPI (Wako). Each section was observed under a fluorescence microscope, LSM710 (Zeiss), with Zen software (Zeiss). Immunohistochemical staining showed colocalization of MYPN and α-actinin in control skeletal muscle. In individuals 2–4, MYPN was not stained, whereas α-actinin showed a normal striated staining pattern. Scale bars represent 10 μm. (B) Each muscle sample was lysed with SDS sample buffer (125 mM Tris-HCl [pH 6.8], 5% 3-mercapto-1,2-propanediol, 2% SDS, and 10% glycerol) and then subjected to western blotting on a NuPAGE 3%–8% Tris–acetate gel (Thermo Fisher Scientific). The primary antibodies used were rabbit polyclonal anti-MYPN (HPA036298; Atlas Antibodies) and mouse monoclonal anti-α-tubulin (DM1A; Sigma-Aldrich). After incubation with secondary antibody, polyvinylidene difluoride membrane was developed with Amersham ECL Western Blotting Detection Reagents (GE Healthcare Life Sciences). Full-length MYPN was not detected in biopsied skeletal muscle from individuals 2–4. α-Tubulin was used as a loading control. Abbreviations are as follows: C1, control 1; C2, control 2; P2, individual 2; P3, individual 3; and P4, individual 4. (C) Western blotting of transdifferentiated myotubes with adenoviral-mediated MYOD1 expression detected MYPN in control cells. A myoblast from individual 1 did not show MYPN. Myosin heavy chain 7 (MYH7) was used as a skeletal muscle differentiation marker, and α-tubulin was used as a loading control. Abbreviations are as follows: C, control; and P1, individual 1.

Figure 4

Skeletal Muscle Studies of Knockin Mice with Heterozygous and Homozygous Nonsense Mutations in Mypn (A) No apparent abnormality was observed in skeletal muscles of wild-type (WT) mice or mutant mice carrying a homozygous MYPN p.Gln526^∗ variant (MYPN^Gln526∗) on H&E and mGT. Scale bar represents 50 μm. (B) Western blotting of MYPN for wild-type (WT), heterozygous (MYPN^WT/Gln526∗), and homozygous (MYPN^Gln526∗) mice. In homozygous mice, full-length or truncated MYPN was not detected. α-Tubulin was used as a loading control. (C) EM of heterozygous (MYPN^WT/Gln526∗) and homozygous (MYPN^Gln526∗) mice. In heterozygous mice, no abnormality in the Z-line was seen. In homozygous mice, Z-line streaming (c) and thickening (d) were observed. In addition, there was a tiny nemaline-like structure that continued from the Z-line (e, inset). Scale bars represent 0.5 μm (a), 1 μm (b), 0.5 μm (c), 0.2 μm (d), 1 μm (e), and 0.2 μm (f).